Digital photography based on CMOS imaging sensors has replaced film for most applications. The typical CMOS imaging sensor consists of an array of pixel cells arranged in a rectangular array of cells having a plurality of rows and columns of pixel cells. Each pixel cell includes a photodiode that converts photons received during an exposure to a charge having a magnitude that is proportional to the light received. The stored charge is coupled to a bit line that is shared by all of the pixel cells in the column. The coupling circuit typically includes four transistors. Three of these are concerned with resetting the photodiode between exposures and coupling the pixel to the corresponding bit line when the pixel is to be read. The fourth transistor is typically connected as a source follower and provides the gain needed to drive the capacitance of the bit line. This transistor does not provide voltage gain. The voltage gain needed to provide a sufficient signal to convert with an analog-to-digital converter is provided by a separate amplifier associated with each of the bit lines.
While this design has provided the basis for numerous successful cameras, the noise levels associated with the design are less than optimal. In high light conditions the noise associated with the pixels is less of an issue since other noise sources, such as the digitization noise introduced by the analog-to-digital converter or shot noise, are larger than the readout noise for each pixel. However, at low light levels, the readout noise becomes significant and limits the lowest light levels that can be utilized to form an image. Ideally, the readout noise in the system should be less than the signal generated by one electron in the photodiode. Each photon that is converted in the photodiode generates one electron; hence, once the readout noise is significantly less than one electron, the noise will be dominated by the statistical noise of electron accumulation. Unfortunately, current CMOS arrays have noise that is of the order of the signal generated by 2-10 electrons RMS, and hence, there is room for significant improvement.